Study On The Effects Of Reef Morphology Variation On Wave Transformation Around Reef Islands

In recent years,due to global climate change and sea level rise,the low-lying atolls fringed by coral reefs are susceptible to coastal inundation during extreme wave events,such as storm surges.The safety of reef-fringed coastal areas and reducing the loss of people and property caused by flood disasters has attracted extensive attention of scholars.It is found that the reef morphology variation is one of the important factors affecting wave run-up at the back-reef beach.So it needs further investigation that atoll with what reef morphology characteristics is more vulnerable to coastal inundation.It is also reported that the low-frequency wave plays an important role in contributing to shoreline wave run-up.Thus,it is necessary to understand the low-frequency wave motions around reef islands.In this thesis,two sets of laboratory experiments were performed in a wave flume to investigate the effects of reef morphology variation on wave transformation around reef islands.The first set of experiments was carried out to study the variations of natural reef morphology(reef flat length,reef surface roughness,reef crest and lagoon).The other set of experiments was to study the effect of reef-flat excavation pit.Subsequently,the numerical model based on Boussinesq equations is validated using the experimental data.It is found that the validated numerical model can simulate the shoreline low-frequency wave and sea-swell wave in the experiment under the effects of reef morphology variations quite well.Finally,the numerical model is employed to further study the effects of the variations of fore-reef slope,reef surface roughness,reef-crest width and lagoon width.The findings in this study can provide some references to prevent reef-fringed coastal areas from coastal inundation.The results show that the total wave height over reef islands decreases all the way from offshore to the back-reef beach due to the wave breaking and reef-surface frictional loss.The sea-swell wave height decreases rapidly over reefs while the low-frequency wave height increases gradually due to the amplification effect of the first reef oscillation mode and reaches its maximum near the shoreline.The reef surface roughness damps both the sea swell height and low-frequency wave height near shoreline dramatically.The existence of reef crest reduce shoreline sea-swell wave height and shoreline low-frequency wave height at the shallow water depth,but promotes the generation of low-frequency wave when the water depth is large.With the increase of the reef crest width,the decrease of sea swell height become slow,but the low-frequency wave has no significant change.The existence of lagoon will reduce the shoreline low-frequency wave height slightly.With the increase of reef flat length,Shoreline sea-swell wave height decreases obviously.The steeper fore-reef slope leads to higher shoreline low-frequency wave height.When the excavation pit exists,shoreline low-frequency wave height is slightly smaller than that of no pit.With the increasing excavation pit width,shoreline low-frequency wave height decreases.Shoreline low-frequency wave height is the largest when the pit is located at the middle of reef flat.